Well, this is not what mech_list (in smtpd.conf) is used for. mech_list
is a list of mechanisms offered by smptd (postfix) to smtp clients. At
the same time, postfix is an ldap sasl client that connects to the ldap
server using ldapdb_mech (digest-md5 in this case). So, this example
_may_ be OK for the openldap authorization.